Effects of suprachiasmatic transplants on circadian rhythms of neuroendocrine function in golden hamsters

Grafts of fetal tissue including the suprachiasmatic nucleus (SCN) of the h ypothalamus restore locomotor rhythmicity to behaviorally arrhythmic, SCN-l esioned Syrian hamsters. We sought to determine whether such transplants al so reinstate endocrine rhythms in SCN-lesioned hamsters. In Exp 1, SCN lesi ons interrupted estrous cycles in a 14 h light, 10 h dark photoperiod and l ocomotor rhythms in constant dim red light (DD). SCN grafts that reinstated behavioral circadian rhythms consistently failed to reestablish estrous cy cles. After ovariectomy, estradiol implants triggered LH surges at approxim ately circadian time 8 in 10 of 12 brain-intact control females and 0 of 9 SCN-lesioned, grafted females. Daily rhythms of the principal urinary melat onin metabolite, 6 alpha-sulfatoxymelatonin, were not reestablished by beha viorally functional grafts. In Exp 2, SCN lesions eliminated locomotor rhyt hmicity in adult male hamsters maintained in DD. Seven to 12 weeks after re storation of locomotor activity rhythms by fetal grafts, hosts and sham-les ioned controls were decapitated at circadian times 4, 8, 12, 16, 20, or 24. Clear circadian rhythms of both serum corticosterone and cortisol were see n in sham-lesioned males, with peaks in late subjective day. No circadian r hythms in either adrenal hormone were evident in serum from lesioned-grafte d males. Testicular regression, observed in intact and sham-lesioned males maintained in DD, was absent not only in arrhythmic SCN-lesioned hamsters g iven grafts of cerebral cortex, but also in animals in which hypothalamic g rafts had reinstated locomotor rhythmicity. The pineal melatonin concentrat ion rose sharply during the late subjective night in control hamsters, but not in SCN-lesioned animals bearing behaviorally effective transplants. Even though circadian rhythms of locomotor activity are restored by SCN tra nsplants, circadian endocrine rhythms are not reestablished. Endocrine rhyt hms may require qualitatively different or more extensive SCN outputs than those established by fetal grafts.